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International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012) © Research India Publications; http://www.ripublication.com/ijaer.htm

Seismic Response of R.C.C Building with Soft Storey

Dr. Saraswati Setia and Vineet Sharma

Associate Professor, Civil Engineering Department NIT, Kurukshetra kurukshetra, India e-mail: [email protected]

Lecturer, Civil Engineering Department. G.P. Nilokheri Haryana, India e-mail: [email protected]

Abstract With urbanization and increasing unbalance of required space to availability, it is becoming imperative to provide open ground storey in commercial and residential buildings. These provisions reduce the stiffness of the lateral load resisting system and a progressive collapse becomes unavoidable in a severe earthquake for such buildings due to soft storey. Soft storey behavior exhibit higher stresses at the columns and the columns fail as the plastic hinges are not formed on predetermined positions. Thus, the vulnerability of soft storey effect has caused structural engineers to rethink the design of a soft storey building in areas of high seismicity. The present analytical study investigates the influence of some parameters on behavior of a building with soft storey. The modeling of the whole building is carried out using the computer program STAAD.Pro 2006. Parametric studies on displacement, inter storey drift and storey shear have been carried out using equivalent static analysis to investigate the influence of these parameter on the behavior of buildings with soft storey. The selected building analyzed through five numerical models. Keywords— Multistory building, Seismic Analysis storey drift, storey shear, soft storey. INTRODUCTION Reinforced-concrete framed structure in recent time has a special feature i.e. the ground storey is left open for the purpose of parking etc. Such building are often called open ground storey buildings or building on stilts. Open ground storey system is being adopted in many buildings presently due to the advantage of open space to meet the economical and architectural demands. But these stilt floor used in most severely damaged or, collapsed R.C. buildings, introduced ‘severe irregularity of sudden change of stiffness’ between the ground storey and upper stories since they had had infilled bricks walls which increase the lateral stiffness of the frame by a factor of three to four times. In such buildings the dynamic ductility demand during probable earthquake gets concentrated in the soft storey and the upper storey tends to remain elastic. Hence the building is totally collapsed due to soft storey effect.

Fig.1 Collapse of a building with soft storey Modica town, in Southern Italy [1] SOFT STOREY BEHAVIOR Many building structure having parking or commercial areas in their first stories, suffered major structural damages and collapsed in the recent earthquakes. Large open areas with less infill and exterior walls and higher floor levels at the ground level result in soft stories and hence damage. In such buildings, the stiffness of the lateral load resisting systems at those stories is quite less than the stories above or below. In Fig.2, the lateral displacement diagram of a building with a soft storey under lateral loading is shown.


Fig.2 Soft storey behavior of a building structure under lateral loading [2] During an earthquake, if abnormal inter-story drifts between adjacent stories occur, the lateral forces cannot be well distributed along the height of the structure. This situation causes the lateral forces to concentrate on the storey (or stories) having large displacement(s). In addition, if the local ductility demands are not met in the design of such a building structure for that storey and the inter-storey drifts are not limited, a local failure mechanism or, even worse, a storey failure mechanism, which may lead to the collapse of the system, may be formed due to the high level of load deformation (P-∆) effects. Fig.3 displays the collapse mechanism of such a building structure with a soft storey under both earthquake and gravity loads.

Fig.3 Collapse mechanism of a building structure having a soft storey [2] Lateral displacement of a storey is a function of stiffness, mass and lateral force distributed on that storey. It is also

known that the lateral force distribution along the height of a building is directly related to mass and stiffness of each story. If the P-∆ effect is considered to be the main reason for the dynamic collapse of building structures during earthquakes, accurately determined lateral displacements calculated in the elastic design process may provide very important information about the structural behavior of the system. Therefore dynamic analysis procedure is required in many of the actual codes for accurate distribution of the earthquake forces along the building height, determining modal effects and local ductility demands efficiently. Although some of the current codes define soft storey irregularity by stiffness comparison of adjacent floors, displacement based criteria for such irregularity determination is more efficient, since it covers all the mass, stiffness and force distribution concepts. DESCRIPTION OF STRUCTURAL MODELS The open ground storey RC buildings exhibit several advantages over conventional moment- resisting frames. However, the structural effectiveness of open ground storey construction is hindered because soft storey effect exhibit higher stress at the column connection and are most likely to fail. In the present work a typical 6 storied RC frame building is being modeled using the computer software STAAD PRO. 2006. The selection of building configuration is basically done as per IS: 456, 2000[4] and the loading details are taken as per IS: 875, 1987 part1 [5] & part2 [6]. The static analysis is then performed for the modeled RC frame building using the computer software STAAD PRO. 2006 and the respective observations are studied. During the development of the analytical models, several issues are taken into consideration.

Fig 4 Plan View of RC Building

In this work it is important to evaluate the existence of the soft storey behavior in this structure. For this reasons two dimensional models are selected for which the soft storey behavior is easily detected. For this a typical rectangular building is taken. Having five bays in X-direction each is of 4.5m span, except the middle one which is of 3.0m span and


the Z-direction there are 3 bays of 4m span each. Height of each story is taken as 3.0m. Five models are generated with this plan of the building by introducing different variation and displacement, story drift, base shear and story shear are the various parameters which are discussed here in this work. In the present study six storied “residential type” open ground RC frame building is considered. The size of the column is 400mmX400mm, 450mmX450mm (for model-3 only ground storey column size is increased) and slab thickness is taken as 200mm for floor slabs as well as for the roof slab. RESULTS AND DISCUSSION The section here deals with the observations and interpretations obtained from the static analysis. Equivalent static analysis is performed for five different models by using the computer software. Model-1 is a bare frame. In model-2 masonry infill panels are introduced in upper floors, model-3 is similar to model-2 with only difference that column size of ground storey is increased by 62% of model-2. Shear walls are introduced in central core and outer periphery in model-4 and 5 respectively to minimize the soft storey effect. The response of any structure is a function of its seismic properties, namely its mass and stiffness. So response of the five models is investigated in terms of displacement and storey shear. Displacement in X-Direction For easy comparison of the lateral displacement of the selected building, plots of the storey level displacement in X-direction versus height are made for the five models, all imposed on the same graph. These are presented in Fig.5. The displacement is inversely proportional to the stiffness. Because Model1 has the smallest stiffness so it has the largest displacement Each model is compared for displacement in X direction. In model-2 the displacement is reduced by 78.73% in comparison to the model-1(bare frame). Also in model-3, 4 & 5 it is reduced by 94.05%, 75.42%, and 96.46% respectively w.r.t to model-1 at top level. The observation shows that the maximum reduction in displacement is in model-5(96.46%) in which a shear wall is introduced in X direction as well as in Z direction. Also model-3(masonry infill in upper floors and with increased size of column of bottom story) shows a good amount of reduction in displacement in X direction. It means the stiffness of the first storey is made within order of equal to the stiffness of the storey above for these two models. With the incorporation of masonry infill in upper floors displacement of bare frame is reduced from 12.935 to 2.751mm. Further the increase in the column size (62% of model-2) of ground storey, lateral displacement is reduced from 2.752 to 0.77mm (approximately 72% reduction in lateral displacement). If we compared the model-2 with model-5, lateral displacement is reduced up to 83% as the shear wall is provided in X-direction as well as in Z-direction.

Displacement in X-direction of Corner Column storey model1 model2 model3 model4 model5

6 12.935 2.751 0.77 3.179 0.457 5 12.045 2.676 0.717 2.658 0.39 4 10.346 2.585 0.647 2.073 0.314 3 8.07 2.482 0.565 1.468 0.231

2 5.449 2.375 0.48 0.887 0.147 1 2.666 2.262 0.4 0.385 0.068

GF 0 0 0 0 0

Fig.5 Displacement in X-Direction

Displacement in Z-Direction For easy comparison of the lateral displacement of the selected building, plots of the storey level displacement in Z-direction versus height are made for the five models, all imposed on the same graph. These are presented in Fig.6. The displacement is inversely proportional to the stiffness. Because Model1 has the smallest stiffness so it has the largest displacement. Each model is compared for displacement in Z-direction. In model-2 the displacement is reduced by 75% in comparison to the model-1(bare frame). Also in model-3, 4 & 5 it is reduced by 92%, 81%, and 95% respectively w.r.t to model-1 at top level. The observation shows that the maximum reduction in displacement is in model-5(95%) in which a shear wall is introduced in X direction as well as in Z direction. Also model-3(masonry infill in upper floors and with increased size of column of bottom story) shows a good amount of reduction in displacement in Z direction. It means the stiffness of the first storey is made within order of equal to the stiffness of the storey above for these two models. Displacement of the building is more in Z-direction in comparison to the X-direction. In model-5 the displacement is increased by 56% in Z-direction in comparison of displacement in X-direction at top level.

Displacement in Z-direction of Corner Column story model1 model2 model3 model4 model5

6 13.59 3.378 1.104 2.527 0.714 5 12.596 3.209 0.989 2.1 0.597 4 10.782 3.011 0.843 1.62 0.468 3 8.383 2.796 0.678 1.135 0.333 2 5.639 2.576 0.509 0.681 0.204 1 2.744 2.36 0.355 0.296 0.089

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Storey shear in X-Direction The parameter which has been considered in this section to study the soft story effect in the building is the storey shear. Storey shear at each level in X-direction for earthquake force in X-direction is obtained for five models (Table III) after performing the analysis on computer program STAAD.Pro 2006. Plots of the storey shear in X-direction versus height are made for the five models, all imposed on the same graph. These are presented in Fig.7. Storey shear in the base of model 2 and 3 is reduced by 76% and 85% when compared with model 1. But it is increased in model 4 and 5 in which a shear wall is used, which is due to increase in the seismic weight of model 4 and 5. Model 3 has minimum storey shear among all five models. Model 1, 4, 5 have a gentle slope (Figure 4.5) which means shear is increasing uniformly, but in model 2, 3 there is an abrupt change storey shear at ground level which means stiffness of the ground storey is less than the storey above.

storey shear in X Direction (kN) storey level Model1 Model2 Model3 Model4 Model5

5 237 7 7 256 270 4 735 20 21 847 821 3 1408 36 38 1648 1616 2 2187 49 54 2584 2387 1 3020 77 66 3602 3253 0 3872 928 582 4687 4159

Fig.7 Storey shearin X-Direction

Storey shear in Z-Direction The parameter which has been considered in this section to study the soft story effect in the building is the storey shear. Storey shear at each level in Z-direction for earthquake force

in Z-direction is obtained for five models (Table IV) after performing the analysis on computer program STAAD.Pro 2006. Plots of the storey shear in Z-direction versus height are made for the five models, all imposed on the same graph. These are presented in Fig.8. Storey shear in the base of model 2 and 3 is reduced by 75% and 86% when compared with model 1. But it is increased in model 4 and 5 in which a shear wall is used, which is due to increase in the seismic weight of model 4 and 5. Model 3 has minimum storey shear among all five models. Model 1, 4, 5 have a gentle slope (Fig.8 ) which means shear is increasing uniformly, but in model 2, 3 there is an abrupt change storey shear at ground level which means stiffness of the ground storey is less than the storey above. Storey shear in Z Direction (kN)

storey level Model1 Model2 Model3 Model4 Model5 5 237 13 9 181 211 4 735 37 28 375 627 3 1408 67 52 601 1158 2 2187 98 76 837 1799 1 3020 122 82 1061 2483 0 3872 973 521 1242 3118

Fig.8 Storey shearin Z-Direction

CONCLUSIONS Lateral displacement is largest in bare frame with soft storey defect both for earthquake force in X-direction as well as in Z-direction for corner columns as well as for intermediate columns. Displacement of intermediate column is more by 0.02% and 0.04% in X and Z-direction respectively w.r.t. corner column. Minimum displacement for corner column is observed in the building in which a shear wall is introduced in X-direction as well as in Z-direction. But in case of intermediate column, displacement is minimum in building having masonry infill in upper floors and with increased column stiffness of bottom story in comparison to the building with shear wall in X-direction as well as in Z-direction. Building having masonry infill in upper floors and with increased column stiffness of bottom story and building with shear wall in core has a small first storey displacement of about 18% and 16% respectively of that of building having masonry infill in upper floors only. This implies that crucial displacement may be effectively reduced if the stiffness of the first storey is made with in the order of magnitude equal to the stiffness of storey above.

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Building with masonry infill in upper floors only shows a sudden change in slope of displacement in X-direction as well as in Z-direction. This is because of abrupt change in storey stiffness. Due to which greater strength is required for first storey columns, which is minimized in building with masonry infill in upper floors and with increased column stiffness of bottom story by increasing the column size of first storey also by incorporating masonry panel in central bay on all four sides. Buildings with shear wall in core and shear wall in X-direction as well as in Z-direction have uniform displacement because of shear wall. Which shows a gradual change of stiffness between the lower soft storey and the upper floors that is essentially required. Buildings having masonry infill in upper floors and with increased column stiffness of bottom story performance well in case of storey shear. Storey shear is minimum in building having masonry infill in upper floors and with increased column stiffness of bottom story amongst of all five models which is 15% of bare frame at first storey. References

[1] Alberto Parducci, Fabrizio Comodini, (2005) “A synergic dissipation approach to retrofit framed structures with a soft first storey,” 9th world seminar on seismic isolation, energy dissipation and active vibration control of structures, Kobe, Japan published.

[2] M. A. Altuntop, “Analysis of building structures with soft stories” Msc dissertation, Dept Civil Eng., Atilim Univ.

[3] V. Sharma, “Seismic response of R.C.C building with soft storey,” Mtech dissertation, Dept Civil Eng, NIT kurukshetra, India.

[4] Indian Standard 456 2000, “Plain and reinforced concrete-code of practice”, India.

[5] Indian Standard 875 1987 part 1, “Code of Practice for design loads (other than earthquakes) for buildings and structures,” India.

[6] Indian Standard 875 1987 part 2, “Code of Practice for design loads (other than earthquakes) for buildings and structures,” India.

[7] Samir Helou, Abdul Razzaq, (2008) “Dynamic behavior of reinforced concrete structures with masonry walls”, An-Najah univ. j. Res (N.Sc) vol. 22.

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